Author Topic: How to adjust air pressure for elevation  (Read 2947 times)

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Offline Little Bill

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How to adjust air pressure for elevation
« on: September 28, 2014, 08:44:28 AM »
Dear all,

I conducted an outdoor measurement by Davis vantage pro 2 in Hanoi. In the setting, I set elevation is 16 meters (according to the internet for elevation of Hanoi). I installed my weather station on top of the high rise building (35 meter height). Now I have 2 question related to the air pressure.
1. I used the elevation is 16 meters for Hanoi in general is correct or not? Or I have to check exactly the point I put my weather station?
2. I want to use the air pressure data to calculate absolute humidity at the single story building. Do I need to covert the air pressure at 35 meter height to air pressure at 0 meter? If I have to convert, which formula I should use for that purpose?

Thank you so much for your kind.

Best regards,

Bill

Offline nincehelser

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Re: How to adjust air pressure for elevation
« Reply #1 on: September 28, 2014, 10:44:43 AM »
I can't tell from your description if the 35m is the height of the building above ground level, or the height of mounting of the outdoor sensor suite above ground level.

You actually want to use the elevation of your barometric sensor, which I assume is in your indoor console.



Offline George Richardson

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Re: How to adjust air pressure for elevation
« Reply #2 on: September 28, 2014, 10:48:19 AM »
Hi Bill,

I will try to help a little but I have to talk in feet and inches. First, barometers are used as altimeters because as you increase your elevation, your station pressure is going to decrease. This happens at a rate of approximately 0.01 inhg/foot. Therefore, you need to calculate the elevation of your VP2 Console (where the barometric pressure is measured) to within about 10 feet if possible.

There are several ways to view "pressure". Your VP2 console will display "Sea Level Pressure", SLP, which factors in "Station Pressure", elevation, and humidity and temperature over the past 12 hours. Most cheaper personal weather stations provide "Altimeter Pressure" because it is instantaneous and takes no complex calculations.

HTH

George


Offline ValentineWeather

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Re: How to adjust air pressure for elevation
« Reply #4 on: September 28, 2014, 12:39:55 PM »
Hi Bill,

I will try to help a little but I have to talk in feet and inches. First, barometers are used as altimeters because as you increase your elevation, your station pressure is going to decrease. This happens at a rate of approximately 0.01 inhg/foot. Therefore, you need to calculate the elevation of your VP2 Console (where the barometric pressure is measured) to within about 10 feet if possible.

There are several ways to view "pressure". Your VP2 console will display "Sea Level Pressure", SLP, which factors in "Station Pressure", elevation, and humidity and temperature over the past 12 hours. Most cheaper personal weather stations provide "Altimeter Pressure" because it is instantaneous and takes no complex calculations.

HTH

George

The sad fact about what Davis has done making their stations report accurate "Sea Level Pressure" is everyone else uses altimeter so those that don't set your Davis station at 0' elevation and match altimeter to a nearby airport will have issues with CWOP if you are part of the Citizen Weather Observer Program.
Randy

Offline miraculon

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Re: How to adjust air pressure for elevation
« Reply #5 on: September 28, 2014, 12:46:14 PM »
Quote
This happens at a rate of approximately 0.01 inhg/foot.

I think you meant 0.001 inHg/foot.

My ASI DBX1 instructions say 1.006" per 1000 ft. (0.001006")

Greg H.


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Offline miraculon

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Re: How to adjust air pressure for elevation
« Reply #6 on: September 28, 2014, 01:05:57 PM »
I just had the epiphany that my Envoys and Consoles in the basement might be different than the two VUE consoles I have. One is on the first floor and the other in a second floor bedroom. Say the difference is 22' or so from the basement to the upper floor.

This should show up as a .02" error.

Sure enough when I brought the upstairs VUE console to the basement and set it to match my ASI DBX1, I could see it change from the set value of 30.14" to 30.11"Hg. A minor difference, but something I never thought about before. As I walked up the stairs I could see the barometer reading change! When I took it back to the basement, it went back to the set value.

Greg H.



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CoCoRaHS: MI-PI-1
CWOP: CW4114 and KE8DAF-13
WU: KMIROGER7
Amateur Radio Callsign: KE8DAF

Offline George Richardson

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Re: How to adjust air pressure for elevation
« Reply #7 on: September 28, 2014, 02:08:46 PM »
Greg,

You are correct, and your follow-up example is exactly what I was processing. If you take a barometer up a flight of stairs (~10'), the displayed pressure will decrease .01 inhg. Sorry for the screw up.

George

Offline Scalphunter

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Re: How to adjust air pressure for elevation
« Reply #8 on: November 04, 2014, 05:23:45 PM »
all altimeters in aircraft  are set to be adjusted  by 01.  0ne inch of  adjustment equals to 1000 foot of altitude. The reading the tower or ATIS give you should match the altitude of the runway. Here where I am the  runway an 100 ft. higher then  the operation tower.  The real fun is keeping it adjusted as you fly any long distance.